Instrument
calibration is vital to a successful control system because signal inputs to
the system controllers come from such instruments. This paper presents a method
for actualizing a standard low-cost loop calibrator for the famous 4-20 mA
electrical signaling scheme. The loop calibrator generates a linear current
signal from 4 to 20 mA over a 250 ? typical process instrument load for
calibration. The realization of the loop calibrator relies on a
voltage-to-current converter to build a constant current source. The voltage
controlled constant current source is built from discrete components and an
op-amp to keep the cost low. Results from simulations and the prototype
demonstrate the performance of the 4-20 mA loop calibrator which utilizes a
greatly reduced number of components. The cost of these components is approximately
34% of the least expensive calibrator sampled, though other production
costs are not included. This conclusion reinforces the fact that loop
calibrators can be cheaper.
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